Together with multi-functionality and miniaturization of electronic equipment such as a multifunctional terminal (smart phone) and a cellular phone, electronic components to be mounted in the electronic equipment also have a trend toward the miniaturization (micro-miniaturization).
For example, the miniaturization advances even in a semiconductor package such as a chip size package (CSP) and the like so that a wafer level semiconductor package, WL-CSP (wafer level CSP) appears. FIG. 1 shows a sectional view of a CSP-typed semiconductor package (CSP package) showing an important portion thereof; and FIG. 2 shows a sectional view of a chip sized WL-CSP semiconductor package (WL-CSP chip) showing an important portion thereof.
In the CSP package 10 shown in FIG. 1, a semiconductor chip 2 mounted on an interposer 1 is molded by any resin while it connects electrodes of the interposer 1 by wire bonding 3 using Au line. 4 indicate this molded body.
A plurality of solder bump electrodes 5 is formed on an undersurface of the interposer 1 and solder balls shown in this figure are connected with these solder bump electrodes 5. CSP package 10 on which plural solder bump electrodes 5 are formed is mounted on a circuit board 7 so that a semiconductor circuit 15 is manufactured.
On the other hand, WL-CSP chip 20 is configured so that the interposer 1 and the molded body 4 are deleted and the plural solder bump electrodes 5 are directly connected with electrodes of the semiconductor chip 2 as shown in FIG. 2.
The CSP package 10 has a package size of about 10×10 mm because the interposer 1 is interposed therein while the WL-CSP chip 20 theoretically has a chip size of (for example, about 4×4 mm). Therefore, the WL-CSP chip 20 can reduce its occupied area substantially on a substrate having a package size, which allows a very high density semiconductor circuit (implementation circuit) to be realized.
By the way, as evaluation properties (evaluation items) of the lead-free solder, which is used for solder balls or the like for joining in WL-CSP chip or the like, wettability (wetting extendability), shear strength properties (shear properties), thermal fatigue resistance (heat cycle characteristics: TCT) and the like are in general listed.
The wettability is a property that is required when forming a solder bump. The shear strength properties are properties that are required to keep strength on a joined interface between solder and each electrode on the substrate. Therefore, the wettability and the shear strength properties are solder properties required when principally joining the lead-free solder to a semiconductor package.
The thermal fatigue resistance is solder property required when principally implementing the semiconductor package on a circuit board. The thermal fatigue resistance is solder property required in a case where it is used in an on-vehicle electronic circuit or the like, which is used under any severe conditions in which there is a considerable change in temperature, or other case. This thermal fatigue resistance is also property to be studied in a case where there is a considerable difference in thermal expansion coefficients between a semiconductor package and a circuit board.
For example, there is an almost twofold difference in thermal expansion coefficients between the CSP package (particularly the interposer 1) and the circuit board (mounting substrate) 7 when mounting the CS package 10 on the circuit board as described above. On the other hand, there is an almost fivefold difference in thermal expansion coefficients between the package (semiconductor chip 2) and the circuit board 7 in WL-CSP chip 20. Accordingly, WL-CSP chip 20 receives any more considerably influence on thermal fatigue resistance of joined solder based on a repeat of heat and cold, which considerably affect reliability of an electronic circuit. Taking this into consideration, lead-free solder which considerably improves its thermal fatigue resistance has been proposed (see Patent Document 1).
The above-mentioned evaluation properties are solder properties required in a case where an electronic circuit is mounted on consumer electronics but in any uses thereof, solderability (solder properties) such as the above-mentioned wettability and shear strength properties is required together with the thermal fatigue resistance. Here, the consumer electronics refer to a cellular phone, a multifunctional terminal (smart phone), a personal computer and the like, other than well-known household electric appliance.